Magnetic tape has been used successfully for many years to store audio, video, computer and other forms of information. A primary goal in the magnetic information storage industry has been that of increased efficiency of storage, i.e., placement of increasingly larger amounts of information on progressively smaller areas of the recording medium. A packing density of 40,000 or more bits per inch of track length has been achieved, and 100,000 or more bits per inch of track length is envisioned for magnetic tapes in the 1990's and beyond.
Modern magnetic tapes generally consist of a polyethylene-terephthalate (PET) film base on which a magnetic material such as cobalt-modified gamma-ferric oxide, generally as a fine powder dispersed in a plastic binder, is deposited on the PET film surface. Information is stored on such tapes as a series of magnetized and unmagnetized domains arranged in a pattern of specific tracks in the magnetic coating. The patterns of magnetized and unmagnetized domains within tracks are established by passing the tapes in proximity to transducers, or "heads," that magnetize the magnetic particles in the tape coating. Analogous transducers may be used to read the pattern of magnetic signals recorded on the tape. A "read" head converts magnetic signals on the tape into electrical signals, and a "write" head converts electrical signals into magnetic signals when recording information on a magnetic tape. So-called "read/write" heads combine these two functions in one structure.
A drive apparatus is required to move the magnetic tape past the read and write heads at a controlled speed and with a minimum of lateral variation in tracking. "Tracking" refers to the proper alignment of the tape with the read and write heads to ensure continuous association of a particular track with the head during the read or write operation. The requirement for consistent tracking is particularly critical in preferred high density applications wherein a plurality of relatively narrow parallel or otherwise congruent tracks with high bit densities are utilized.
Refinements for increased recording efficiency in high density tape applications have involved the use of progressively narrower tracks and progressively smaller spaces between tracks. With helical drives, one or more rotating magnetic heads engage a tape wrapped at least partially around a cylindrical member such that the tape path describes a portion of a helix. Tracks generated by the heads in a helical drive thereby track diagonally across the tape. Generally, the recorded tracks lie at an angle substantially less than 45 degrees with reference to the longitudinal axis of the tape. The space between the centerlines of two adjacent tracks generated by "helical drives," for example, may be as small as about 12 micrometers.
In general, the combination of a moving tape with a spinning head wheel provides a more rapid recording and read-out of information than is possible with movement of tape past a stationary head wheel. Given the desirability of a spinning head wheel, the geometry of helical drives and resultant diagonal tracking across the tape allows for more efficient utilization of the surface area of the tape than is possible with non-helical drives.
In one typical helical drive configuration, four read/write heads are mounted on a scanning head wheel that is rotated as the tape passes the recording or reading areas of the heads. Each of the four heads is separated from its neighboring heads by 90 degrees on the head wheel. Rotation of the head wheel and movement of the tape is adjusted such that when the preceding head is on the middle of the track length (that is, in the middle of the tape width), the following head is on the beginning of its track. In other words, one half of the track represents one-fourth of the head wheel revolution.
A representative arrangement of parallel, diagonally oriented tracks produced by a helical drive is depicted schematically in FIG. I. The diagonal tracks 5 are generally confined to a central area on the surface of the tape 16, as depicted in FIG. 1. The lateral margins of the tape may be reserved for various forms of reference data, such as cue tracks, audio controls and time codes.
Given the ability to record information on large numbers of congruent tracks having very small centerline-to-centerline separations, it is possible to pack large amounts of information on relatively small areas of tape. On the other hand, such high packing densities mandate great accuracy in any tape drive mechanism having the task of precisely aligning the directional movement of read and write heads with individual tracks. Even slight deviations of the read/write head from the two-dimensional orientation of an individual track may cause the position of the head to overlap onto an adjacent track. The result is that information on adjacent tracks is read inappropriately as "noise" superimposed on the information readout from the correct track. This can lead to partial or even complete loss of the information read-out from the magnetic tape.
Helical recorders will generally stress the tape on the input and output guides so that a bias is created that forces the tape to rest firmly on the bottom edge guides. This is required to maintain the tight tolerance required by the narrow helical recorded tracks. Hence, when tapes recorded on helical recorders are viewed with uniform longitudinal stress, the tracks will be curved. This indicates the differential non-uniform stress that the tape was subjected to during the recording.
Tracking errors due to misalignment of the read/write heads with the tracks on a magnetic tape are minimized when the same read/write heads and the same drive mechanism are used to read the tape as were used to record on the tape. For example, the track laid down by a read/write head may be slightly out of line or otherwise imperfect due to mechanical imperfections in the drive and/or head wheel structure. Nevertheless, if the very same drive/head wheel structure is used to read the tracks as was used to record the tracks (assuming the same mechanical imperfections are present in each operation), the same pattern of tracking errors as were produced in the "write" or record mode may be duplicated in the "read" mode. In other words, if the same head wheel and drive are used in both operations, the read heads likely will be able to follow individual recorded tracks without deviating onto adjacent tracks. The result would be a correct "read" of the information on the magnetic tape in spite of inherent tracking errors.
On the other hand, tracking errors during recording may lead to substantial or even catastrophic loss of information if the tape is run in a machine that is unable to duplicate the original track configurations. For example, tracking errors may arise from differing manufacturing tolerances in alignments of the tape drives and scanning head wheels of different machines. Even with the same machine, field use and changes in stability of the drive and scanning mechanisms over time may render magnetic tapes previously produced by the machine to be "unreadable" at a later date. Moreover, magnetic tapes themselves may undergo deformation due to stretching, aging, temperature changes, humidity changes and other variables, irrespective of structural variation in the recording/reading machines.